Implicit signalling in ofdm preamble with embedded signature sequence, and cyclic prefix and postfix aided signature detection

ABSTRACT

A transmitter transmitting payload data using Orthogonal Frequency Division Multiplexed (OFDM) symbols, including: a frame builder configured to receive the payload data and to receive signalling data to use in detecting and recovering the payload data at a receiver, and to form the payload data with the signalling data into frames for transmission: a modulator configured to modulate a first OFDM symbol with the signalling data and to modulate one or more second OFDM symbols with the payload data; a signature sequence processor circuit providing a signature sequence; a combiner circuit combining the signature sequence with the first OFDM symbol; a prefixing circuit prefixing a guard interval to the first OFDM symbol to form a preamble; and a transmission circuit transmitting the preamble and the one or more second OFDM symbols. The guard interval is formed from time domain samples of a part of the signature sequence.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 14/897,468, which is a 35 U.S.C. § 371 application of InternationalApplication No. PCTiGB2014/051922, filed on Jun. 24, 2017, which claimspriority to UK Patent Application No. 1312048.0 (filed on Jul. 4, 2013),1403392.2 (filed on Feb. 26, 2014), and 1405037.1 (filed on Mar. 20,2014). The entire disclosures of the prior applications are herebyincorporated by reference in their entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates to transmitters, receivers and methods oftransmitting and receiving payload data using Orthogonal FrequencyDivision Multiplexed (OFDM) symbols.

BACKGROUND OF THE DISCLOSURE

There are many examples of radio communications systems in which data iscommunicated using Orthogonal Frequency Division Multiplexing (OFDM).Television systems which have been arranged to operate in accordancewith Digital Video Broadcasting (DVB) standards for example, use OFDMfor terrestrial and cable transmissions. OFDM can be generally describedas providing K narrow band sub-carriers (where K is an integer) whichare modulated in parallel, each sub-carrier communicating a modulateddata symbol such as for example Quadrature Amplitude Modulated (QAM)symbol or Quadrature Phase-shift Keying (QPSK) symbol. The modulation ofthe sub-carriers is formed in the frequency domain and transformed intothe time domain for transmission. Since the data symbols arecommunicated in parallel on the sub-carriers, the same modulated symbolsmay be communicated on each sub-carrier for an extended period. Thesub-carriers are modulated in parallel contemporaneously, so that incombination the modulated carriers form an OFDM symbol. The OFDM symboltherefore comprises a plurality of sub-carriers each of which has beenmodulated contemporaneously with different modulation symbols. Duringtransmission, a guard interval filled by a cyclic prefix of the OFDMsymbol precedes each OFDM symbol. When present, the guard interval isdimensioned to absorb any echoes of the transmitted signal that mayarise from multipath propagation.

As indicated above, the number of narrowband carriers K in an OFDMsymbol can be varied depending on operational requirements of acommunications system. The guard interval represents overhead and so ispreferably minimized as a fraction of the OFDM symbol duration in orderto increase spectral efficiency. For a given guard interval fraction,the ability to cope with increased multipath propagation whilstmaintaining a given spectral efficiency can be improved by increasingthe number K of sub-carriers thereby increasing the duration of the OFDMsymbol. However, there can also be a reduction in robustness in thesense that it may be more difficult for a receiver to recover datatransmitted using a high number of sub-carriers compared to a smallernumber of sub-carriers, because for a fixed transmission bandwidth,increasing the number of sub-carriers K also means reducing thebandwidth of each sub-carrier. A reduction in the separation betweensub-carriers can make demodulation of the data from the sub-carriersmore difficult for example, in the presence of Doppler frequency. Thatis to say that although a larger number of sub-carriers (high orderoperating mode) can provide a greater spectral efficiency, for somepropagation conditions, a target bit error rate of communicated data mayrequire a higher signal to noise ratio to achieve than required for alower number of sub-carriers.

As will be appreciated, therefore providing an arrangement in whichsignaling data conveying information for the detection of payload datacarrying OFDM symbols can represent a significant challenge.

SUMMARY OF THE DISCLOSURE

Various further aspects and embodiments of the disclosure are providedin the appended claims, including but not limited to a transmitter fortransmitting payload data using Orthogonal Frequency DivisionMultiplexed (OFDM) symbols, the transmitter comprising a frame builderconfigured to receive the payload data to be transmitted and to receivesignalling data for use in detecting and recovering the payload data ata receiver, and to form the payload data with the signalling data intoframes for transmission. The transmitter also comprising a modulatorconfigured to modulate a first OFDM symbol with the signalling data andto modulate one or more second OFDM symbols with the payload data, asignature sequence circuit for providing a signature sequence, acombiner circuit for combining the signature sequence with the firstOFDM symbol, a prefixing circuit for prefixing a guard interval to thefirst OFDM symbol to form a preamble, and a transmission circuit fortransmitting the preamble and the one or more second OFDM symbols. Thecombiner is configured to combine the signature sequence with the firstOFDM symbol, and the guard interval is formed from time domain samplesof a part of the signature sequence.

Embodiments of the present technique can be arranged to form the guardinterval of the OFDM symbol carrying the signalling data to includesamples formed only from a part of a signature sequence, the OFDM symboland the guard interval forming a preamble for the one or more secondOFDM symbols. By arranging for the guard interval of the OFDM symbolcarrying the signalling data to include samples formed only from a partof a signature sequence, there is an increased likelihood of a receiverdetecting the signature sequence, for example using a matched filter.Furthermore by combining the signature sequence with the OFDM symbolcarrying the signalling data, then an accuracy of channel impulseresponse estimation at the receiver using the signature sequence iscorrespondingly increased.

In accordance with the present technique a transmitter is adapted toform a preamble which comprises an OFDM symbol carrying signalling datawhich forms part of a transmission frame. In order to detect a signaturesequence which is combined with the preamble and to allow decoding ofthe signalling data in presence of inter-channel interference caused bya significant echo path, embodiments of the present technique arrangefor the guard interval of the preamble to be formed entirely fromsamples which are formed from a part of the signature sequence which hasbeen copied from the time domain samples which are combined with thefirst OFDM symbol carrying the signalling data. The signature sequencemay be added at a lower power to the time domain samples of the OFDMsymbol carrying the signalling data. Accordingly, correlation in thetime domain to detect a channel impulse response may include parts ofthe signature sequence present throughout the preamble.

Furthermore, in order to improve a likelihood of correctly detecting thesignalling data, in one example, the post fix samples are formed fromanother part of the signature sequence which the part of the signaturesequence which forms the guard interval. The samples of one part of thesignature sequence which forms the guard interval/pre-fix and the sampleof the other part of the signature sequence which forms the post-fix aretherefore different part of the signature sequence or part thereof whichis combined with the first OFDM symbol carrying the signalling data.With such an arrangement a significant echo path which may causeinter-channel interference at the receiver can be cancelled bysynthesising and removing the part of the post fix which causesinter-channel interference in the wanted samples of the OFDM symbolcarrying the signalling data.

According to the embodiments of the present technique therefore, apreamble may be formed from a first OFDM symbol carrying the signallingdata, a guard interval which forms a pre-fix generated from only timedomain samples of the signature sequence which is also combined at areduced power level with the OFDM symbol carrying the signalling dataand a post fix formed from another part of the time domain samples ofthe signature sequence which is combined with the first OFDM symbol toform the guard interval.

In some examples, the transmitter can chose from amongst a group ofsignature sequences which signature sequence it uses and a receiver maydetect from the guard interval which signature sequence has beencombined with the first OFDM symbol. Consequently a message conveyed bythe choice of signature sequence may be detected from the guard intervalonly and without having to detect the content of the first OFDM symbol.

In another embodiment the signature sequence may be produced by either apseudo random binary sequence generator, an M-sequence generator or aGold code sequence generator.

The use of such binary sequences allows differential matched filteringof the received guard intervals to be performed without reducing theaccuracy of the signature sequence detection. The use of differentialencoding allows matched filtering to be utilised for framesynchronisation or preamble detection when a frequency offset is presentin the received signal.

In another embodiment the message provided by a selection of thesignature sequence is an indication of an early warning signal (EWS).

Utilising the choice of conveyed signature sequence to carry an EWSmessage enables OFDM receivers to quickly, reliably and efficientlydetect an EWS and therefore provide an EWS and related information to auser of the receiver. Performing EWS detection on the guard intervalallows a low complexity approach to detection of a EWS which may beperformed whilst the receiver is in a reduced power or standby state.This therefore allows EWS detection to be performed intermittentlywithout consuming a large amount of power.

In another embodiment there is provided a receiver for detecting andrecovering payload data from a received signal, the receiver comprisinga detector circuit for detecting the received signal. The receivedsignal comprises the payload data, signalling data for use in detectingand recovering the payload data, the signalling data being carried by afirst Orthogonal Frequency Division Multiplexed (OFDM) symbol, and thepayload data being carried by one or more second OFDM symbols, and thefirst OFDM symbol having been combined with the signature sequence andprefixed with a guard interval comprising a part of the signaturesequence, the symbol being followed by post fix samples which are formedfrom another part of the signature sequence which forms the guardinterval, to form a preamble.

The receiver also comprises a synchronisation circuit comprising amatched filter, and a demodulator circuit for recovering the signallingdata from the first OFDM symbol for recovering the payload data from thesecond OFDM symbols. The matched filtering circuit comprises a guardinterval duration matched filter, the guard interval duration matchedfilter having an impulse response matched to a differentially encodedpredetermined portion of time domain samples of the signature sequence.The effect of the matched filtering is that an output of the guardinterval duration matched filter generates a signal representing acorrelation of the differentially encoded predetermined portion of timedomain samples of the signature sequence and a differentially encodedportion of the received signal corresponding to the guard interval. Thistherefore allows the matched filtering circuit to detect the signaturesequence from which the guard interval of the received signal has beenformed and with which the first OFDM symbol has been combined. In thismanner the receiver may detect from the guard interval which signaturesequence has been combined with the first OFDM symbol and the channelimpulse response.

In one example a message conveyed by the signature sequence may bedetected from the guard interval without having to detect and processthe whole preamble. This therefore reduces the processing required at areceiver in order to establish which signature sequence has beentransmitted, thus decreasing processing times and complexity withregards to detecting a conveyed message.

Various further aspects and features of the present disclosure aredefined in the appended claims, which include a method of transmittingpayload data and a method of detecting and recovering payload data.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present disclosure will now be described by way ofexample only with reference to the accompanying drawing in which likeparts are provided with corresponding reference numerals and in which

FIG. 1 provides a schematic diagram illustrating an arrangement of abroadcast transmission network;

FIG. 2 provides a schematic block diagram illustrating an exampletransmission chain for transmitting broadcast data via the transmissionnetwork of FIG. 1;

FIG. 3 provides a schematic illustration of OFDM symbols in the timedomain which include a guard interval;

FIG. 4 provides a schematic block of a typical receiver for receivingdata broadcast by the broadcast transmission network of FIG. 1 usingOFDM;

FIG. 5 provides a schematic illustration of a sequence of transmissionframes for transmitting broadcast data and payload data separated by apreamble carrying signalling data;

FIG. 6 provides a block diagram showing a transmitter for transmittingsignalling data via a signalling or preamble OFDM symbol;

FIG. 7 provides a schematic diagram showing a receiver for receivingsignalling data via signalling or preamble OFDM symbol;

FIG. 8 provides a schematic diagram showing a transmitter fortransmitting signalling data via a signalling or preamble OFDM symboland transmitting a message via a choice of signature sequence;

FIG. 9 provides a schematic diagram showing a transmitter fortransmitting signalling data via a signalling or preamble OFDM symboland transmitting a message via a choice one of two signature sequences;

FIG. 10 provides a schematic diagram showing a transmitter fortransmitting signalling data via a signalling or preamble OFDM symboland transmitting a message via a choice of signature sequence in apreamble OFDM symbol as may be conceived in the time domain;

FIG. 11 provides a schematic diagram showing a preamble for transmittingsignalling data and a message via a choice of signature sequence in apreamble OFDM symbol;

FIG. 12 provides a diagram showing a possible sequencing of transmissionand reception of early warning signals transmitted via a choice ofsignature sequence in a preamble OFDM symbol;

FIG. 13 provides a schematic diagram showing a transmitter in accordancewith an embodiment of the present technique for transmitting signallingdata via a signalling or preamble OFDM symbol and transmitting a messagevia a choice of signature sequence in a guard period of a preamble OFDMsymbol as may be conceived in the time domain;

FIG. 14 provides a schematic diagram showing a preamble in accordancewith an embodiment of the present technique for transmitting signallingdata and a message via a choice of signature sequence in a guard periodof a preamble OFDM symbol;

FIG. 15 provides a schematic diagram showing a transmitter in accordancewith an embodiment of the present technique for transmitting signallingdata via a signalling or preamble OFDM symbol and transmitting a messagevia a choice of signature sequence in a guard period of a preamble OFDMsymbol;

FIG. 16 provides a schematic diagram showing a receiver in accordancewith an embodiment of the present technique for receiving signallingdata via a signalling or preamble OFDM symbol and receiving a choice ofsignature sequence message in a guard period of a preamble OFDM symbol;

FIG. 17 provides a schematic diagram of a differential guard intervalmatched filter in accordance with an embodiment of the presenttechnique;

FIG. 18 provides a schematic diagram of a differential encoder;

FIG. 19 provides a schematic diagram of a differential guard intervalmatched filter in accordance with an embodiment of the presenttechnique;

FIG. 20 provides a schematic diagram showing a transmitter in accordancewith an embodiment of the present technique for transmitting signallingdata via a signalling or preamble OFDM symbol and transmitting a messagevia a choice of signature sequence in a guard period of a preamble OFDMsymbol as may be conceived in the time domain;

FIG. 21 provides a schematic diagram showing a preamble in accordancewith an embodiment of the present technique for transmitting signallingdata and a message via a signature sequence in a guard period of apreamble OFDM symbol;

FIG. 22 provides a schematic diagram showing a preamble OFDM symbol inaccordance with an embodiment of the present technique for transmittingsignalling data and a signature sequence in a guard period;

FIG. 23 is an illustrative representation of example preamble OFDMsymbols which are combined at a receiver as a result of passing througha channel impulse response having a significant echo path to form areceived signal at the receiver;

FIG. 24 is a schematic representation of the example preamble OFDMsymbols formed into a received signal as represented in FIG. 23illustrating a formation of inter-channel interference and noise fromthe signature sequence;

FIG. 25 provides a schematic diagram showing a transmitter in accordancewith an embodiment of the present technique corresponding to the exampleshown in FIG. 15, and also including a post-fix insertion circuit;

FIG. 26 provides a schematic diagram showing a preamble in accordancewith an embodiment of the present technique for transmitting signallingdata and a signature sequence in a guard period of a preamble OFDMsymbol and including a post-fix formed from the samples of the guardinterval by the transmitter of FIG. 25; and

FIGS. 27a to 27d provide an illustrative representation of an exampleoperation of a_receiver which uses a post-fix of the preamble OFDMsymbol to recover signalling data through the use of an FFT; and

FIG. 28 provides a schematic diagram showing a preamble in accordancewith an embodiment of the present technique for transmitting signallingdata and a signature sequence in a guard period of a preamble OFDMsymbol and including a post-fix formed from the samples of the signaturesequence which are different to those from which the guard interval isformed.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Embodiments of the present disclosure can be arranged to form atransmission network for transmitting signals representing dataincluding video data and audio data so that the transmission networkcan, for example, form a broadcast network for transmitting televisionsignals to television receiving devices. In some examples the devicesfor receiving the audio/video of the television signals may be mobiledevices in which the television signals are received while on the move.In other examples the audio/video data may be received by conventionaltelevision receivers which may be stationary and may be connected to afixed antenna or antennas.

Television receivers may or may not include an integrated display fortelevision images and may be recorder devices including multiple tunersand demodulators. The antenna(s) may be inbuilt to television receiverdevices. The connected or inbuilt antenna(s) may be used to facilitatereception of different signals as well as television signals.Embodiments of the present disclosure are therefore configured tofacilitate the reception of audio/video data representing televisionprograms to different types of devices in different environments.

As will be appreciated, receiving television signals with a mobiledevice while on the move may be more difficult because radio receptionconditions will be considerably different to those of a conventionaltelevision receiver whose input comes from a fixed antenna.

An example illustration of a television broadcast system is shown inFIG. 1. In FIG. 1 broadcast television base stations 1 are shown to beconnected to a broadcast transmitter 2. The broadcast transmitter 2transmits signals from base stations 1 within a coverage area providedby the broadcast network. The television broadcast network shown in FIG.1 may operate as a so called multi-frequency network where eachtelevision broadcast base stations 1 transmits its signal on a differentfrequency than other neighbouring television broadcast base stations 1.The television broadcast network shown in FIG. 1 may also operate as aso called single frequency network in which each of the televisionbroadcast base stations 1 transmit the radio signals conveyingaudio/video data contemporaneously so that these can be received bytelevision receivers 4 as well as mobile devices 6 within a coveragearea provided by the broadcast network. For the example shown in FIG. 1the signals transmitted by the broadcast base stations 1 are transmittedusing Orthogonal Frequency Division Multiplexing (OFDM) which canprovide an arrangement for transmitting the same signals from each ofthe broadcast stations 2 which can be combined by a television receivereven if these signals are transmitted from different base stations 1.Provided a spacing of the broadcast base stations 1 is such that thepropagation time between the signals transmitted by different broadcastbase stations 1 is less than or does not substantially exceed a guardinterval that precedes the transmission of each of the OFDM symbols thena receiver device 4, 6 can receive the OFDM symbols and recover datafrom the OFDM symbols in a way which combines the signals transmittedfrom the different broadcast base stations 1. Examples of standards forbroadcast networks that employ OFDM in this way include DVB-T, DVB-T2and ISDB-T.

An example block diagram of a transmitter forming part of the televisionbroadcast base stations 1 for transmitting data from audio/video sourcesis shown in FIG. 2. In FIG. 2 audio/video sources 20 generate theaudio/video data representing television programmes. The audio/videodata is encoded using forward error correction encoding by anencoding/interleaver block 22 which generates forward error correctionencoded data which is then fed to a modulation unit 24 which maps theencoded data onto modulation symbols which are used to modulate OFDMsymbols. Depicted on a separate lower arm, signalling data providingphysical layer signalling for indicating for example the format ofcoding and modulation of the audio/video data is generated by a physicallayer signalling unit 30 and after being encoded by an encoding unit 32,the physical layer signalling data is then modulated by a modulationunit 24 as with the audio/video data.

A frame builder 26 is arranged to form the data to be transmitted withthe physical layer signalling data into a frame for transmission. Theframe includes a time divided section having a preamble in which thephysical layer signalling is transmitted and one or more datatransmission sections which transmit the audio/video data generated bythe audio/video sources 20. A symbol interleaver 34 may interleave thedata which is formed into symbols for transmission before beingmodulated by an OFDM symbol builder 36 and an OFDM modulator 38. TheOFDM symbol builder 36 receives pilot signals which are generated by apilot and embedded data generator 40 and fed to the OFDM symbol builder36 for transmission. An output of the OFDM modulator 38 is passed to aguard insertion unit 42 which inserts a guard interval and the resultingsignal is fed to a digital to analogue convertor 44 and then to an RFfront end 46 before being transmitted by an antenna 48.

As with a conventional arrangement OFDM is arranged to generate symbolsin the frequency domain in which data symbols to be transmitted aremapped onto sub carriers which are then converted into the time domainusing an inverse Fourier Transform which may comprise part of the OFDMmodulator 38. Thus the data to be transmitted is formed in the frequencydomain and transmitted in the time domain. As shown in FIG. 3 each timedomain symbol is generated with a useful part of duration Tu seconds anda guard interval of duration Tg seconds. The guard interval is generatedby copying a part of the useful part of the symbol with duration Tg inthe time domain, where the copied part may be from an end portion of thesymbol. By correlating the useful part of the time domain symbol withthe guard interval, a receiver can be arranged to detect the start ofthe useful part of the OFDM symbol which can be used to trigger a FastFourier Transform to convert the time domain symbol samples into thefrequency domain from which the transmitted data can then be recovered.Such a receiver is shown in FIG. 4.

In FIG. 4 a receiver antenna 50 is arranged to detect an RF signal whichis passed via a tuner 52 and converted into a digital signal using ananalogue to digital converter 54 before the guard interval is removed bya guard interval removal unit 56. After detecting the optimum positionfor performing a fast Fourier Transform (FFT) to convert the time domainsamples into the frequency domain, an FFT unit 58 transforms the timedomain samples to form the frequency domain samples which are fed to achannel estimation and correction unit 60. The channel estimation andcorrection unit 60 then estimates the transmission channel for exampleby using pilot sub-carriers which have been embedded into the OFDMsymbols. After excluding the pilot sub-carriers, all the data-bearingsub-carriers are fed to a symbol de-interleaver 64 which de-interleavesthe sub-carrier symbols. A de-mapper unit 62 then extracts the data bitsfrom the sub-carriers of the OFDM symbol. The data bits are fed to a bitde-interleaver 66, which performs the de-interleaving so that the errorcorrection decoder can correct errors in accordance with a conventionaloperation.

Framing Structure

FIG. 5 shows a schematic diagram of the framing structure of a framethat may be transmitted and received in the systems described withreference to FIGS. 1 to 4. FIG. 5 illustrates different physical layerframes, some targeted for mobile reception whilst others are targetedfor fixed roof-top antenna reception. The system can be expanded infuture to incorporate new types of frames, for the current system, thesepotential new types of frames are simply known as future extensionframes (FEFs).

One requirement for fixed reception frames is an improved spectralefficiency which may be assured by such features as adopting a higherorder modulation, for example 256QAM, and higher code rates, for examplegreater than half rate, because of relatively benign channel conditions,and a high number of sub-carriers per OFDM symbol (FFT size) such as32K. This reduces the capacity loss due to the guard interval fraction.However, a higher number of sub-carriers can make such OFDM symbolsunsuitable for mobile reception because of lower tolerance to highDoppler frequency of the received signal. On the other hand, the mainrequirement for mobile reception frames could be robustness in order toensure a high rate of service availability. This can be improved byadopting such features as a low order modulation for example QPSK orBPSK, low code rates, a low number of sub-carriers per OFDM symbol (FFTsize) and a high density scattered pilot pattern etc. A low number ofsub-carriers for OFDM symbols can be advantageous for mobile receptionbecause a lower number of sub-carriers can provide a wider sub-carrierspacing and so more resilience to high Doppler frequency. Furthermore ahigh density pilot pattern eases channel estimation in the presence of atime varying propagation channel.

The framing structure shown in FIG. 5 is therefore characterised byframes which may each include payload data modulated and encoded usingdifferent parameters. This may include for example using different OFDMsymbol types having different number of sub-carriers per symbol, whichmay be modulated using different modulation schemes, because differentframes may be provided for different types of receivers. However eachframe may include at least one OFDM symbol carrying signalling data,which may have been modulated differently to the one or more OFDMsymbols carrying the payload data. Furthermore for each frame, thesignalling OFDM symbol may be a different type to the OFDM symbol(s)carrying the payload data. The signalling data is required to berecovered so that the payload data may be de-modulated and decoded.

Frame Preamble

To delimit frame boundaries, a frame preamble symbol such as the P1symbol in DVB-T2 is required. The preamble symbol would carry signallingthat describes how the following frame is built. It is expected that allof the types of receiver mentioned above whether for mobile or fixedreception should be able to detect and decode the preamble in order todetermine whether or not they should decode the payload in the followingframe.

The preamble OFDM symbol conveys signalling data whilst the OFDM symbolswithin the body of the transmission frame convey payload data as shownin FIG. 5. Each transmission frame shown in FIG. 5 has particularcharacteristics. A data bearing frame 100 carries a frame of data, whichmay use a higher operating mode providing a higher number ofsub-carriers per OFDM symbol, for example, approximately 32 thousandsub-carriers (32 k mode) thereby providing a relatively high spectralefficiency, but requiring a relatively high signal to noise ratio toachieve an acceptable data integrity in the form of the bit error rate.The higher order operating mode would therefore be most suitable tocommunicate to stationary television receivers which have sensitivedetection capabilities including well positioned fixed antenna forrecovering audio/video data from the 32 k OFDM symbols. In contrast, theframe structure also includes a second frame 102 which is generated tobe received by mobile television receivers in a more hostile radiocommunications environment. The frame 102 may therefore be arranged toform payload bearing OFDM symbols with a lower order modulation schemesuch as BPSK or QPSK and a small or lower number of sub-carriers perOFDM symbol (FFT size) such as 8K to improve the likelihood that amobile receiver may be able to receive and recover the audio/video datain a relatively hostile environment. In both the first frame 100 and thesecond frame 102 a preamble symbol 104,106 is provided which providessignalling parameters for detecting the audio/video data transmitted inthe payload part of the transmission frame 100, 102. Similarly, apreamble symbol 108, 110 is provided for a future extension frame 112.

In the Applicant's co-pending UK patent application 1305795.5,arrangements for forming a preamble symbol for use with the transmissionframes of FIG. 5 are disclosed. The disclosed preambles result in animproved likelihood of detecting the preamble symbol particularly inharsh radio environments. Furthermore, the framing structure shown inFIG. 5 can be devised such that the number of sub-carriers of thepayload bearing OFDM symbols is different from frame to frame andfurthermore, these sub-carriers may use different modulation schemes.Thus the OFDM symbols which carry the payload data may be of a differenttype to the OFDM symbols carrying the signalling data. An example blockdiagram of a part of the transmitter shown in FIG. 2 for transmittingthe preamble and signalling data as disclosed in UK patent application1305795.5 is shown in FIG. 6.

In FIG. 6 the signalling data is first fed to a scrambling unit 200which scrambles the signalling data which is then fed to a forward errorcorrection (FEC) and modulator unit 202 which encodes the signallingdata with a forward error correcting code and then interleaves it beforemapping the encoded data onto QAM modulation symbols. The QAM modulationcould be π/4-BPSK, QPSK, 16QAM, 64QAM or 256QAM for example. A pilotinsertion unit 204 then inserts pilots in between modulation symbols toform the OFDM symbol of the preamble 104, 106, 108, 110. The OFDM symbolforming the preamble is then scaled by a scaling unit 206 in accordancewith a predetermined factor (1−G). The scaling unit 206 adapts thetransmission power of the preamble with respect to a signature sequencewhich is combined with the OFDM symbols of the preamble beforetransmission so that the total transmission power of the preambleremains the same as it would have been without the signature sequence.The signature sequence generator 208 is configured to generate asignature sequence which is fed to a second scaling unit 210 whichscales the signature sequence by a predetermined factor G before thescaled signature sequence is combined with the OFDM symbol of thepreamble by a combining units 212. Thus the signature sequence W(k) iscombined with the OFDM symbol in the frequency domain so that each ofthe coefficients of the signature sequence is added to one of thesubcarrier signals of the OFDM symbol. The combined preamble OFDM symboland signature sequence are then transformed from the frequency domain tothe time domain by an inverse Fourier transform processor (IFFT) 214before a guard interval insertion unit inserts a time domain guardinterval. At an output of the guard insertion unit 216 the preamblesymbol is formed on output channel 218.

As can be seen for the example shown in FIG. 6 the signature sequence iscombined with the OFDM symbol carrying signalling data in the frequencydomain so that a spectrum of the preamble symbol after combining remainswithin a spectral mask for the transmission channel. As will beappreciated for some examples the signature sequence may be combinedwith the OFDM symbol in the time domain. However other bandwidthlimiting processes may then be required to be applied to the signaturesequence prior to combination with the preamble OFDM symbol in the timedomain which may affect the correlation properties of the signaturesequence at the receiver.

In the example illustration in FIG. 6, the scrambling of the signallingdata by the scrambling unit 200 ensures that the peak-to-average powerratio (PAPR) of the preamble symbol will not be excessive due to manysimilarly modulated OFDM sub-carriers. The scrambled signalling bits arethen forward error correction encoded by the FEC and BPSK unit 202 witha LDPC code at a low code rate prior to mapping to a low orderconstellation within the unit 202. Although BPSK is specified in FIG. 6,a range of other modulation schemes may also be used, for example a formof QAM may be used. The pilots inserted at this stage by the pilotinsertion unit 204 are not for channel estimation, but for frequencyoffset estimation as will be explained shortly. At this stage, a complexpreamble signature sequence also composed of the same number of complexsamples as the useful sub-carriers as the OFDM symbol is added to thesamples of the signalling OFDM symbol by the combiner 212. Aftergeneration and before addition to the preamble OFDM symbol, eachpreamble signature sequence sample is scaled by a predetermined factorG, by a scaler 210 and the corresponding OFDM symbol sample is scaled by(1-G) by a scaler 206 so that the power of the composite preamble symbolshould be the same as the power of the signalling OFDM symbol at point Ain FIG. 6.

The IFFT 214 then forms the OFDM symbol in the time domain, which isthen followed by the insertion of the guard interval by the guardinsertion unit 216 which in some examples prepends the Ng samples of thepreamble OFDM symbol at the start of the preamble OFDM symbol—also knownas a cyclic prefix of the preamble OFDM symbol. After guard intervalinsertion, a preamble OFDM time domain symbol of duration Ts=Tu+Tg madeup of Ns=Nu+Ng complex samples where Tu is the useful symbol period withNu samples and Tg is the guard interval duration with Ng samples isformed.

As explained above, the preamble symbol generator of FIG. 6 generates asignature sequence which is combined with the signalling OFDM symbol(first OFDM symbol of a physical layer frame), which forms the preamblesymbol of the frame, in order to allow a receiver to detect the preambleat lower signal to noise ratios compared to the signal to noise ratioswhich are required to detect and recover data from OFDM symbols carryingthe payload data. The formation of the signature sequences generated bythe signature sequence generator 208 is also disclosed in the Applicantsco-pending UK patent application 1305795.5. Each signature sequence maybe formed from a pair of Gold code sequences chosen because of theirgood auto-correlation properties, or in other example signaturesequences formed from M-sequences could be used. In still other examplesthe sequences may be pseudo random binary sequences which are formedfrom linear feedback registers. Further detail of the selection of thesesequences and their formation into signature sequences can be found inthe Applicant's co-pending UK patent application 1305795.5 where thefollowing example generator polynomials for the definition of the linearfeedback register for the real and imaginary components are also given.

TABLE 1 Generator polynomials for complex signature sequence. SequenceName Generator polynomial R_seq1 (I₀(x)) x¹³ + x¹¹ + x + 1 R_seq2(I₁(x)) x¹³ + x⁹ + x⁵ + 1 I_seq1 (Q₀(x)) x¹³ + x¹⁰ + x⁵ + 1 I_seq2(Q₁(x)) x¹³ + x¹¹ + x¹⁰ + 1

For the linear feedback shift registers the initialising values for theshift register stages to initiate generation of each of the sequencesG₀(n) or G₁(n) at the start of each preamble symbol are presented in thetable below:

Sequence I or Q Initialisation (LSB first) G₀(n) I 1111111111111 Q1110111011111 G₁(n) I 0110110110111 Q 0101010101010

As shown in FIG. 6, the scaler 210 multiplies the signature sequence bya factor G and the scaler 206 multiplies the signalling OFDM symbol by afactor 1−G. As such, if the time domain signalling OFDM symbol signal isc(n) while the signature sequence signal is f(n), then the compositetransmitted preamble symbol s(n) is given by:

s(n)=(1−G)c(n)+Gf(n)

where G is the scaling applied to the signature sequence. The signaturesignal effectively adds distortion to the signalling OFDM symbol therebyincreasing the bit error rate of the signalling OFDM symbol at thereceiver. Furthermore, with a normalised power of 1, the compositesymbol in effect distributes power between the signature sequence signaland the signalling OFDM symbol signal. With a high value for G, thesignature signal has more power and so frame synchronisation (detectionof the preamble) at the receiver should be achieved at a lower signal tonoise ratio. However, reducing the power of the signalling OFDM symbol(in order to increase the power of the signature sequence signal) alsomeans that error-free decoding of the signalling information itselfbecomes more difficult at the receiver as the signal-to-noise of thesignalling OFDM symbol has fallen. Therefore, an optimum value for G hasto be a compromise between these conflicting aims. We can further defineP=(1−G)/G which is proportional to the power ratio between thesignalling OFDM symbol and the signature signal. An appropriate valuefor G can be set by experimenting with this power ratio P.

Determining a Suitable Guard Interval Fraction

According to example embodiments of the present technique, the samepreamble symbol will delimit physical layer frames meant for both fixedand mobile reception. In the following analysis it is assumed that abroadcast transmission system, which has both types of transmissionframes will be used. As such one of the principal factors affecting thereception of payload data bearing OFDM symbols transmitted for fixedreception is spectral efficiency. As explained above, this means the useof large numbers of sub-carriers for the OFDM symbols andcorrespondingly large FFT sizes because a smaller guard intervalfraction (GIF) can be used to get a large guard interval duration (GID).A large GID can allow a broadcast system to have a greater separationbetween broadcast transmitters and can operate in environments with agreater delay spread. In other words the broadcast transmission systemis configured with a wider spacing between transmitters forming a largersingle frequency network (SFN). A detailed analysis of the selection ofa suitable guard interval fraction can be found in the Applicant'sco-pending UK patent application 1305795.5 where the following possibleguard interval fractions were proposed. In a 6 MHz channel raster systemin which for example DVB-T2 is transmitted, an OFDM symbol havingsubstantially four thousand sub-carriers (4K) OFDM symbol has a durationof only 2*224*8/6=597.33 us. On the other hand, the longest delay spreadin the network is 709.33 us (the longest GID for 32K, 19/128 GIF) evenwith a GIF=1, it is not possible to get a GID of 709.33 us with a 4KOFDM symbol. A table below lists possible operating modes that arereceivable in medium to high Doppler frequencies (for the mobileenvironment) and some possible guard intervals. Accordingly, for thisexample an appropriate signalling OFDM symbol is an 8K symbol with a GIDof 19/32, but in other examples a GIF of 57/128 may be used so that theresulting GID is equivalent to that of a 32 k symbol with a GIF of57/512.

TABLE 2 Mobile FFT modes and their possible guard intervals FFT Tu in GMTs Size 6 MHz (us) GIF (us) (us) 4K  597.33 1 597.33 1194.667 ¼ 298.671493.338 8K 1194.67 ½ 597.33 1792.005 19/32 709.33 1904.000 ¾ 896.002090.638

Frequency Offset Considerations

At first detection, the signalling or preamble OFDM symbol may have tobe decoded in the presence of any tuning frequency offsets introduced bytuner 52. This means that either the signalling data should be modulatedonto the preamble OFDM symbol in a manner that reduces the effects ofany frequency offsets or resources are inserted into the preamble symbolto allow the frequency offset to be estimated and then removed prior topreamble decoding. In one example the transmission frame may onlyinclude one preamble OFDM symbol per frame so the first option isdifficult to achieve. For the second option, additional resources can bein the form of frequency domain pilot subcarriers, which are insertedinto the OFDM so that these can be used to estimate the frequency offsetand common phase error. The frequency offsets are then removed beforethe symbol is equalised and decoded. In a similar vein to the insertionof pilots into the data payload bearing OFDM symbols, embodiments of thepresent technique can be arranged to provide within the signalling(preamble) OFDM symbol pilot sub-carriers, which can allow for theestimation of frequency offsets that are larger than the preamblesubcarrier spacing. These pilots are not spaced regularly in thefrequency dimension to avoid instances when multipath propagation mayresult in regular nulls of the pilots across the full preamble OFDMsymbol. Accordingly, 180 pilot sub-carriers can be provided across the8K symbol with the positions defined apriori. The sub-FFT bin frequencyoffset is estimated via the detection of the preamble OFDM symbolitself. Accordingly embodiments of the present technique can provide apreamble OFDM symbol in which the number of sub-carriers carrying pilotsymbols is less than the number which would be required to estimate achannel impulse response through which the preamble OFDM symbol istransmitted, but sufficient to estimate a coarse frequency offset of thetransmitted OFDM symbol.

Preamble Detection and Decoding at the Receiver

A portion of a receiver which is for the reception and detection of theabove described signalling data that is combined with a signaturesequence is illustrated in FIG. 7, where all numerical values are forexample only. The receiver comprises four main processing stages orelements 701 to 704, each of which provides information required for theoperation of a subsequent processing stage. A signal received from anantenna is converted to a sampled time domain baseband signal and inputvia an input 705 into the first processing stage 701, which comprises aguard interval correlator 706. The guard interval correlator 706correlates the guard interval of the preamble with a portion of thereceived signal in order to obtain a fine frequency offset estimationbut also a coarse symbol time which is used to determine the startingpoint in time of the fast Fourier transform (FFT) window of the nextprocessing stage 702. Within processing stage 702 a fast Fouriertransformer 707 performs an FFT on the received signal which has beenstored in memory 708, where the start point of the FFT window isadjusted according to the coarse symbol timing obtained by the guardinterval correlator 706. The frequency domain signal out from theFourier transformer is input to a signature sequence correlator 709which obtains a coarse frequency offset in terms of FFT bins. Thiscoarse frequency offset information is combined with the fine frequencyoffset information obtained by the guard interval correlator to arriveat a frequency offset ‘A’. The frequency offset A is then used tocorrect the frequency offset present in the received signal via acomplex multiplication 711 between an output of a local oscillator 710and the received baseband signal during processing stage 703. Thefrequency corrected received signal is then matched filtered by asignature sequence matched filter 712, which is populated with thecoefficients of the predetermined time-domain version of the signaturesequence, and a channel impulse response extracted from the outputsignal by a channel impulse response (CIR) extractor 713. The CIR isthen transformed into the frequency domain by a Fourier transformer 714and used for equalisation. The signature sequence matched filter 712also provides a fine preamble symbol timing which is then used tocorrectly position the FFT window of the Fourier transformer 715 whichtransforms the frequency corrected received signal into the frequencydomain. The fine preamble symbol timing is indicated by an impulselike-peak in the output of the matched filter 712 where further detailon the implementation of the matched filter may be found in theApplicant's co-pending UK patent application 1305795.5. Frequency domainequalisation is then performed on the frequency domain received signalby the equaliser 716 using the channel transfer function derived byapplying the FFT 714 to the CIR. Finally the signature sequence isremoved from the received signal by a signature sequence remover 717 andthe signalling data decoded from the preamble by a preamble decodingunit 718.

signature Sequence Messages

As disclosed in the Applicant's co-pending UK patent application1305795.5, as well as utilising the signature sequence for the provisionof a CIR and fine preamble symbol timing in harsh radio environments,the choice of signature sequence may also be used to convey informationor a message. For instance, by selecting a signature sequence from a setof signature sequences the selection of the signature sequence mayconvey information, such as an indication of a presence of an absence ofan active early warning signal (EWS) within the signalling data orpayload. This indication may be received at the receiver by detectingwhich signature sequence from the set of possible signature sequenceshas been combined with the signalling data.

FIG. 8 illustrates the transmitter presented in FIG. 6 but with anadaptation to enable the signature sequence to convey additional data ormessages. Since the transmitter of FIG. 8 is based on the transmitterillustrated in FIG. 6 where like parts have the same reference numerals,only the differences shall be explained.

As illustrated in FIG. 8, the signature sequence generator 208 formspart of a signature sequence processor 800, which includes the signaturesequence generator 208 a sequence number controller 804. The input 802to the signature sequence generator 208 receives the output from thesequence number controller 804. The sequence number controller input 806represents the message that the transmitter would like to convey toreceivers within the network. The signature sequence generator 208 isconfigured to be able to generate one of N+1 possible sequences. A givennumber 0≤i≤N on the input 802 of the signature sequence generator 208causes the signature sequence generator 208 to output the sequence whosecardinal number is i from amongst its set of signature sequences. Theoutput of one or other of the signature sequences from generator 208conveys a pre-determined message to all receivers in the network thatreceive the signal. In one example the message represents an EWS.

The signature sequence generated by the signature sequence generator 208is one of a predetermined set of sequences which represent as manymessages as there are signature sequences generated by the signaturesequence generator 208. In order to communicate each of these messages,the message number of input 806 is arranged to be the required signaturesequence number which the signature sequence generator 208 uses toselect one of the signature sequences from its predetermined set ofsignature sequences. The selection of the signature sequence istherefore representative of a different one of a correspondingpredetermined set of messages which thereby conveys information whichmay be a particular warning message, such as a tsunami warning or may bea message for a different purpose. Each message can provide differentinformation. For example in a N=4 message system, message 1 could be anearly warning of a possible emergency situation, such as an approachinghurricane or tsunami while message 2 could be an indication of anall-clear prior to the normal state represented by message 0 whichrequires no particular action. The early warning signal could triggerthe receiver to display a message or audible warning instructing usersof the device to evacuate a building for example. Thus a receiver coulddetect the message 1 and generate audible or visual output to the usersto provide a warning. Similarly messages message 3 and message 4 couldprovide similar broadcast information, such as public safetyannouncement, radio traffic announcements or flooding. As will beunderstood, the choice of sequence thereby represents one of themessages selected and therefore conveys information.

For example, when there is need to convey an EWS to all receivers, theinput 806 to the signature sequence processor 800 carries a 1.Accordingly, the sequence number controller 804 outputs ‘1’ onto input802 of the signature sequence generator 208 which causes the signaturesequence generator 208 to generate an ‘EWS On’ signature sequence whichis one of the set of signature sequences and output this to the gainblock 210. When there is no EWS to be conveyed, the input 806 to thesignature sequence processor 800 carries a ‘0’. Accordingly, thesequence number controller 804 outputs ‘0’ onto input 802 of thesignature sequence generator 208 which causes the signature sequencegenerator 208 to generate an ‘EWS Off’ signature sequence which is oneof the set of signature sequences and output this to the gain block 210.In this example, all receivers within the network detecting signaturesequence corresponding to input ‘1’ and the ‘EWS On’ signature sequencesdetermine that this represents an EWS, further information about whichmay be carried as part of the Layer 1 signalling data and/or in thepayload of the frame. The receiver can then take further action todecode and interpret the emergency information. On the other hand,receivers detecting signature sequence number zero would determine thatthere are no current emergencies imminent and so continue to decode anddisplay the audio-visual information in the payload of the frame.

FIG. 9 illustrates the transmitter presented in FIG. 8 which has beenadapted for operation with an EWS ON and OFF signal. The signaturesequence W(k) which is added to the signalling OFDM symbol by the adder212 is output from a multiplexer 901 and may either be a signaturesequence generated by a signature sequence generator 902 or a signaturesequence generated by a signature generator 903, however the differentsignature sequences may also be generated by a single generator. Thecontrol of which signature sequence is passed to the adder is providedby the EWS unit which indicates to the multiplexer which signaturesequence to pass to the adder. For example, if there is an impendingdisaster the EWS unit 904 would provide a signal to the multiplexerwhich configures the multiplexer to provide a signature sequence whichindicates the presence of a EWS in the signalling or payload data

FIGS. 8 and 9 show the insertion of one of a set of signature sequencesin the frequency domain. FIG. 10 provides a schematic diagram ofsignature sequence insertion elements of a transmitter when thesignature sequence insertion is performed in the time domain accordingto the operation disclosed in the Applicants co-pending UK patentapplication 1305795.5. The signalling OFDM symbol has been transformedinto the time domain to form the symbol 1001. The symbol 1001 is thenscaled by the scaling factor (1−G) by a time domain scalar or multiplier1002. The scaled signal is then added or combined to a time domainsignature sequence signal by the adder or combiner 1003. As in FIG. 9, amultiplexer 1004 under the control of a EWS signal supplied by an EWSunit 1005 is utilised to supply the different signature sequences to thescalar or multiplier 1006 which scales the signature sequence by G.Depending on the presence of a EWS signal, either the time domainsignature sequence represented by 1007 (EWS Off) or 1008 (EWS On) iscombined with the signalling symbol 1001 to form the final time domainpreamble. As shown in FIG. 10, the time domain signature sequences andsignalling data have a conventional cyclic prefix or guard intervalintroduced prior to combining and therefore a dedicated guard intervalunit is not required after the signature sequence has been combined withthe signalling symbol. However, a time domain guard interval processormay therefore be required prior to the combiner in order to introduce aguard interval in the signalling symbol and the signature sequencesprior to combining.

FIG. 11 provides a schematic illustration of the preamble in the timedomain once the signalling data symbol has been combined with asignature sequence by either of the transmitter elements of FIGS. 8 to10. In FIG. 11 G<1 and therefore the signature sequence is at asignificantly lower power than the signalling data.

Improved Messaging Arrangement with Signature Sequences

As described above, in order to receive a message conveyed by theselection of a signature sequence, a receiver needs to detect whichsignature sequence from a set of signature sequences has been combinedwith the signalling symbol. For example, if signature sequence 1 of aset of two signature sequences is detected this may indicate forinstance that there is an active emergency warning signalling in thesignalling data or payload data. The determination of the signaturesequence combined with the signalling data symbol may be performed inprocessing stage 703 of the receiver of FIG. 7 where the signaturesequence matched filtering takes place. In examples where only a singlesignature sequence may be combined with signalling data symbol only asingle matched filter is required. However, where more than onedifferent signature sequences may be transmitted a matched filtercorresponding to each of the possible signature sequences is required.Peak/pulse detection or thresholding may then be performed on the outputof each of the matched filters to detect which signature sequence hasbeen transmitted. Thus if a peak is detected in the output of a matchedfilter corresponding to signature sequence 1, it can be determined thatsignature sequence 1 was combined with the signalling symbol and themessage conveyed by the selection of signature sequence 1 is received.

Although performing transmission and reception in the above describedmanner enables additional information to be conveyed by the signaturesequence, there may be a number of associated disadvantages. The use ofsignature sequences to convey an indication of a message enables an EWSto be rapidly distributed to a wide range of devices. However, in termsof earthquake warning signals for example, the time taken for an EWS tobe received and decoded is critical because of the short period of timebetween an earthquake warning (arrival of the primary seismic wavesP-waves) and the arrival of the destructive secondary seismic waves(S-waves). Consequently, the position of the signature sequencedetection at the third stage of the receiver of FIG. 7 may introduceunacceptable delays. A second case of a delay which is introduced intothe EWS reception process, is that the EWS indication is only beingconveyed by the preamble of a frame. For instance, as shown in FIG. 12,imagine that the detection of an impending earthquake takes place at aseismograph station at time 1201, just after preamble 1202.Consequently, an EWS will not be transmitted until the next preamble1203 is transmitted, thus a significant delay may have been introduced.It is critical that this time is reduced in order to reduce the overalltime for detection of the EWS signal after, for example, primary waves(P-waves) are detected. In order to reduce this time, the period betweenpreambles may be reduced. For instance, in some earthquake early warningsystems a maximum period of time between detection of an earthquake 1201and the detection of an EWS at a receiver 1205 is specified, for example100 ms as shown in FIG. 12. Consequently, in this example the frequencyof preamble transmission should be equal to or preferably below 100 msso that the period of time between 1201 and 1206 is below 100 ms.Furthermore, as described above there will also be a subsequent delay1204 until the EWS signal is detected 1205 at the receiver because ofthe processing delays inherent in all the processes before the matchedfiltering of the signature sequences in the receiver of FIG. 7. In orderto reduce this delay it may be advantageous to also reduce theprocessing time before detection of the transmitted signature sequenceat the receiver. A reduction in processing time at the receiver may alsoin some examples allow the time between preambles to be increased andtherefore system capacity increased whilst remaining within thespecified EWS detection time limits.

As well as the temporal disadvantages associated with the previouslydescribed techniques and receivers, there may also be disadvantagesassociated with the complexity of the processing required at thereceiver and power consumption at the receiver. Firstly, in processingstage 703 a matched filter is required for each signature sequence ofthe set of signature sequences and therefore increased memory and anincreased number of arithmetic operations will be required as the set ofsignature sequences increases. For example, the signature sequences maybe 8192 samples in length, therefore if there are two signaturesequences in the set of signature sequence double this number ofarithmetic and memory elements may be required. Secondly, inapplications such as broadcast television, it may be required that atelevision continues to receive and monitor transmitted signals formessages such as an EWS even if the television is in standby mode.Consequently, it would be beneficial to reduce the power consumption ofthe receiver when it is solely detecting which signature sequence hasbeen transmitted. In the previously described receiver, the processingstages 701, 702 and 703 are required to operate if the television is tomonitor for EWS even though when in standby mode many of the processesperformed in processing stages 701 to 703 are redundant because theyproduce information for the reception and decoding of the signalling andpayload data. Consequently, if the received signals are to becontinually monitored for the presence of an EWS-on signature sequence,it may also be beneficial in terms of energy consumption if the laterprocessing stages of the receiver were not required to be operational.Energy saving measures such as these may also be beneficial in scenarioswhere software updates are transmitted during periods where a receiversuch as a television set is in a standby mode. For instance, atelevision set could be instructed to exit standby mode when anappropriate signature sequence is transmitted and begin decodingsignalling and payload data in order to receive the software updates.Energy saving measures such as these may also be highly beneficial inscenarios where the TV receiver is a battery powered device such as amobile phone.

Guard Interval Matched Filtering

FIG. 13 provides a schematic diagram of the equivalent time domainprocessing in a transmitter in accordance with an embodiment of presenttechnique. The elements of the transmitter of FIG. 13 are substantiallysimilar to those of FIG. 10, however, the signals and symbols which theycombine differ. Time domain signalling data symbol 1301 no longerincludes a guard interval. Instead a guard interval is provided by thesignature sequence which is to be combined with the signalling datasymbol to form a preamble. As can be seen in FIG. 13, the guard intervalof the signature sequences is formed as a cyclic prefix by replicatingan end portion of the respective signature sequence and placing it atthe front of the signature sequence. When an indication of an EWS is tobe transmitted, the signature sequence signal 1303 is combined with thesignalling symbol 1301 and when an indication of an EWS is not to betransmitted, the signature sequence signal 1302 is combined with thesignalling symbol 1301. A consequence of this new arrangement of theguard interval and signature sequences is that an indication of whichsignature has been combined with the signalling data symbol is presentin the guard interval. Consequently, as is explained in further detailbelow, only the fraction of the signature sequence in the guard intervalis required to be detected in order to establish which signaturesequence from a set of signature sequences has been combined withsignalling symbol. Although the above described embodiment has beendescribed with regard to EWS, the message conveyed by the selection ofsignature sequence and therefore the guard interval may be of anyappropriate sort, for instance an automatic start-up command or anindication that software updates are to be transmitted.

FIG. 14 provides an illustration of the preamble formed by thetransmitter of FIG. 13 when an EWS-off signature sequence is transmitted1401 and when an EWS-on signature sequence is transmitted 1402. It canbe seen that the guard intervals 1403 and 1404 of the preambles areformed from a portion of the signature sequence as opposed thesignalling data as is the case in the transmitter described withreference to FIG. 10.

FIG. 15 provides a frequency domain implementation of the transmitter inaccordance with the embodiment illustrated in FIG. 13. The elements ofthe transmitter are substantially similar to those of FIG. 9, however,there are a number of differences arising from the need to form theguard interval from the selected signature sequence. In particularinverse Fourier transformers 1501 1502 are required to transform thesignalling symbol and the selected frequency domain signature sequenceinto the time domain prior to insertion of guard intervals by the guardinterval inserters 1503 1504. However, as shown in FIGS. 13 and 14, insome examples a cyclic prefix may not be inserted in to the guardinterval preamble symbol.

Another difference of the transmitter shown in FIG. 15, with respect tothe example shown in FIG. 13, is that the gain of the respective samplesof the signature signal and the samples of the cyclic prefix of the OFDMsymbol in the guard interval and the equivalent gains within the usefulpart of the preamble symbol which is carrying the signalling data may beset independently. Accordingly the gain for the samples in the guardinterval and the useful part of the symbol are set with respect tofactors Q and P. As such, for example, samples which are formed fromcombining the cyclic prefix of the time domain 8K preamble OFDM symbolc(n) with the samples of the time domain signature sequence, which maybe for example one of the signature sequences g₀(n) or g₁(n), may beexpressed for each sample of the guard interval s(n) for n=0, 1, . . . ,Ng−1 as the following equation:

s(n)=√{square root over ((1−P))}g _(x)(N _(u) −N _(g) +n)+√{square rootover (P)}c(N _(u) −N _(g) +n) for 0≤n<N _(g)

Whereas the samples of the OFDM symbol carrying the signalling data(useful part of the OFDM symbol) may be expressed for each sample as theequation:

s(n)=√{square root over (Q)}g _(x)(n−N _(g))+√{square root over((1−Q))}c(n−N _(g)) for N _(g) ≤n<N _(s)

Where g_(x) implies either of g₀ or g₁ and for example P and Q are closeto zero, Nu=8192, Ng=3648 and Ns=Nu+Ng=11840. In one example, the factorQ is set such that 10 log[(1−Q)/Q]=10 dB while P is set such that 10log[(1−P)/P]=8 dB=G. This means that (Q, P)=(0.090909091, 0.136806889).In yet another example P is set such that P=0.

According to the above expressions for the samples of the signaturesequence and those of the OFDM symbol carrying the signalling data, therelative gain for guard interval samples are √P for the OFDM cyclicprefix and √{square root over (1−P)} for the signature sequence cyclicprefix, while the gain provided by the scaler 206 for the OFDM symbolssamples is √{square root over (1−Q)} and the scaler 210 for thesignature sequence is √{square root over (Q)}. When P=0, the preambleguard interval only contains samples of the cyclic prefix of thesignature sequence and none from the cyclic prefix of the OFDM symbol.

In one example therefore the preamble guard interval may have the sameduration as the longest possible guard interval in the system being57/512 for the 32K OFDM symbol. This is equivalent to a 57/128 guardinterval fraction for the 8K preamble OFDM symbol. This guard intervaltherefore comprises 8192* 57/128=3648 samples. These samples are formedfrom combining the cyclic prefix of the time domain 8K preamble OFDMsymbol c(n) to that of either of the time domain signature sequencesg₀(n) or g₁(n).

Example Receiver

FIG. 16 provides a schematic diagram of a receiver in accordance with anembodiment which is configured to receive preambles described withreference to FIGS. 14 and 15. A number of the elements of the receiverof FIG. 16 are substantially similar to those previously described withreference to FIG. 7 and for brevity only those that differ shall bedescribed. The received baseband signal is first input into processingstage 1601 and a differential guard interval matched filtering unit1601. As will be explained below, differential encoding is used inconjunction with the matched filter in order to reduce the effects offrequency offsets in the received signal. Within the differential guardinterval matched filtering unit 1602, samples of the relevant portions(i.e., the portions which were used to form the guard interval or cyclicprefix) of the differentially encoded signature sequences are utilisedto populate the taps of the guard interval duration matched filters,where there is a matched filter corresponding to each signature sequencefrom the set of signature sequences. The differentially encoded receivedsignal is then filtered by each of the matched filters and a peak of asufficient amplitude or a greatest amplitude at the output of one of thefilters indicates that the a portion of the signature sequencecorresponding to that filter has been detected and therefore that it isthe corresponding signature sequence that was combined with thesignalling symbol. In FIG. 16 this indication is denoted as a messageindicator 1603. In addition to outputting a message indicator, thedifferential matched filtering unit 1602 also estimates the coarsesymbol timing and fine frequency offset required in later stages of thereceiver. In processing stage 1604 of the receiver, only a singlematched filter procedure is required, where the matched filtercorresponds to the signature sequence detected by the differential guardinterval matched filtering unit 1602. Apart from the use of only asingle matched filter at processing stage 1604, the processing withinprocessing stage 1604 corresponds to that described with reference toprocessing stage 703 in FIG. 7.

As described above, the matched filtering has two purposes. Firstly, itprovides the fine frequency offset and coarse symbol timing estimateswhich are required to position the Fourier transform window and performfrequency offset correction later in the receiver, respectively.Secondly, performing the matched filtering allows the receiver todetermine which signature sequence has been transmitted prior tosignature sequence matched filtering that occurs in element 1602 of thereceiver. As well as providing an indication of the signature sequencewhich has been combined with the signalling data symbol earlier than theexisting transmission and reception methods, the receiver of FIG. 16also utilises reduced length matched filters compared to those in stage1602 because the length of the matched filter is only required to matchthe number of samples in the guard interval rather than the numbersamples in the entire preamble.

Differential Guard Interval Matched Filtering

In the embodiments of the present technique, guard interval matchedfiltering replaces the guard interval correlation at processing stage1601. However, matched filtering is not immune to frequency offsets inthe received signal. Consequently, if a signature sequence which formsthe guard interval is to be detected, a coarse symbol timing obtained,and a fine frequency offset measured, means to overcome the frequencyoffset in the received signal is required. As is known in the art,differential encoding a signal removes any frequency offset present asignal. Consequently, in accordance with some embodiments and asdescribed above, this is achieved by differential encoding of thereceived signal and the signature sequences of the set of signaturesequences prior to the guard interval matched filtering.

FIG. 17 provides a schematic illustration of one of the differentialguard interval matched filters that forms the differential guardinterval matched filtering unit 1602. The received baseband signal isdifferentially encoded by the differential encoder 1704 and one of thesignatures sequences from the set of signature sequences is generated bythe signature sequence generator 1701 and transformed into the timedomain by the inverse Fourier transform unit 1702. The time domainsignature sequence is then differentially encoded by the differentialencoder 1703. The differentially encoded received signal is then matchfiltered by a filter whose taps correspond to the samples of relevantportion of the differentially encoded time domain signature sequence.This process is performed for each of the signature sequences of the setof signature sequences and the presence of a particular signaturesequence is determined by detecting a peak in the output of theappropriate matched filter. The position of the peak in the signaloutput from the matched filter also indicates the coarse symbol timingand the argument of the peak indicates the fine frequency offset of thereceived signal.

FIG. 18 provides an illustration of a differential encoder 1704 or 1703.An input signal such as the received signal or a portion of a signaturesequence is delayed by one sample by a delay element 1801 and a secondversion of the input signal is conjugated by the conjugator 1802. Therespective signals output by 1801 and 1802 are then multiplied by acomplex multiplier 1803 to produce a differential encoded version of theinput signal.

FIG. 19 provides a schematic illustration of an example of processingstage 1601 which may form a part of an embodiment where the messageconveyed by the guard interval is the presence or absence of a EWS andEWS related data in the signalling and payload data. The sampledbaseband received signal is first differential encoded as previouslydescribed and it is then matched filtered by two matched filters 1901and 1902, which are matched to a differentially encoded portion of eachof the signature sequences that correspond to the guard interval of thepreamble. The output from each of the matched filters is input to acomparator 1903 which indicates to a demultiplexer whether a EWS signalis present or not, but also outputs a fine frequency offset and a coarsesymbol timing based on positions of peaks in the signals output from thedifferential matched filters 1901 and 1902. If a signature sequenceindicating ‘EWS on’ is detected the receiver will commence EWSprocessing where data in the signalling or the payload data is detectedand appropriately processed, for displaying on a TV screen for example.If a signature sequence indicating ‘EWS off’ is detected the receivermay continue with the processing of the received signal as recited abovewith reference to FIG. 16 if the television or reception apparatus iscurrently being used. Alternatively, if the device is in a standby-modethe receiver will not proceed with decoding the remainder of thereceived signal and—the receiver would go back to standby to wake up atthe time when the next preamble is expected.

FIG. 20 provides a schematic diagram of a time domain functional view ofthe transmitter in accordance with an example embodiment. The structureof this functional view of the transmitter is substantially similar tothat illustrated in FIG. 13 but the signals that are combineddifferently. In a previous embodiment the guard interval of the preamblewas completely formed from a portion of the chosen signature sequence.However, in this embodiment the guard interval is formed from both aportion of the signature sequence and the signalling symbol, where thesignalling symbol in the guard interval is at a lower amplitude comparedto the portion of signature sequence. In accordance with the explanationprovided above with respect to the operation of the transmitter shown inFIG. 15, the gain provided to each of the samples of the signaturesignal/sequence and the samples of the OFDM signalling symbol are√{square root over (Q)} and √{square root over (1−Q)} respectivelyduring the useful part of the symbol and the gain of the guard intervalsamples for the signature signal and the OFDM signalling symbol arerespectively √{square root over (1−P)} and √{square root over (P)}.

In some existing OFDM systems, a cyclic prefix or guard interval isformed from a portion of the useful symbol and so a slight misplacementof the FFT window due to inaccurate timing information does notsignificantly impact upon the decoding accuracy of the data contained inthe FFT window. This robustness arises because any portion of thesignalling cut off from the end of the symbol is also contained in theguard interval and therefore will still be captured by the misplaced FFTwindow. However, in the previous embodiment functionally illustrated inFIGS. 13 and 14, misplacement of the FFT window due to multipathpropagation may result in inter-carrier interference (ICI) amongst thedata sub-carriers of the signalling OFDM symbol thereby degradingdecoding accuracy because a portion of the signalling OFDM symbol is nolonger repeated in the guard interval. Consequently, in the presentembodiment, by introducing a portion of the signalling OFDM symbol intothe guard interval the adverse effects of FFT window misplacement can bereduced. Although the amplitude of the signalling OFDM symbol in theguard interval is comparatively low, it has been shown that thisimproves a decoding accuracy of the signalling OFDM symbol. Furthermore,the low amplitude of the guard interval signalling data, allows thedifferential guard interval matched filtering of the received signal tobe unaffected by the samples of the signalling OFDM symbol, thusmaintaining the receiver's ability to detect which signature sequencehas been transmitted and the associated message indicator.

FIG. 21 provides an illustration of preambles that may be formed by thetransmitter of FIG. 20 when the message conveyed by the signaturesequence selection is the presence or absence of a EWS. As can be seenfrom FIG. 21, the preambles 2101 and 2102 each include a guard period2103 and 2104 which are formed primarily from portions of the signaturesequences 2105, 2106 but also from portions of the signalling OFDMsymbol 2107, 2108. In some examples, the signalling OFDM symbol portionof the guard interval may have an amplitude of −8 dB compared to thesignature sequence and the signature sequence and signalling OFDM symbolmay be formed from 8 k OFDM symbols which have approximately 6912 usefulsubcarriers. Furthermore, the guard interval may be 57/128 of 8192samples in length, therefore having a length of 3648 samples. Althoughthese parameters are suitable for 8 k OFDM symbol, the parameters aremerely example parameters and may vary depending on othercharacteristics of system, for example the separation betweentransmitters and the required capacity of the system.

In receivers disclosed in the Applicant's co-pending UK patentapplication 1305795.5, constant amplitude zero autocorrelation (CAZAC)sequences were proposed as a suitable sequences for the signaturesequences. However, in embodiments differential encoding of CAZACsequences can reduce a likelihood of correctly detecting the symboltiming and the signature sequence from which a guard interval is formed.

Further Example Embodiments

Further example embodiments of the present technique will now bedescribed with reference to FIGS. 22 to 28. In accordance with theexamples shown in FIG. 14, in one example, the samples of the guardinterval are generated entirely from the signal samples or part of thesignal samples of the signature sequence. Thus in FIG. 22, incorrespondence with the parameters for generating the signature sequenceshown in FIG. 15 and explained above, the value of P=0, so that theguard interval part 2201 does not include any component from the OFDMsignalling symbol 2203, which is usually generated according to a cyclicprefix. The value of Q can be set to any value (Q=a) in order to varythe component of the signature sequence which is added to the OFDMsymbol carrying the signalling data. Thus the OFDM signalling symbol2205 includes a component from the samples of the signature signal 2202.According to this example there is an improved likelihood of detectingthe signature sequence because the guard interval does not include anysamples of the OFDM symbol.

As will be explained shortly in other examples the samples of thesignature sequence present in the guard interval can be cancelled fromthe OFDM symbol carrying the signalling data in the presence of an echopath which may cause inter-channel interference. Such an effect of anecho path is illustrated in FIG. 23.

In FIG. 23, an OFDM symbol formed as a preamble carrying the useful datawhich in this case is the signalling data 2301 comprises a componentformed from the signature sequence 2302 and samples formed from theuseful part of the OFDM symbol 2304. The same preamble sequence is thenshown as if transmitted by a second path 2306 in respect of a first path2308. The two paths 2306, 2308 are formed by a channel impulse response2310. The effect is to delay the transmission of the preamble withrespect to the first path 2308 so that a portion 2310 of the guardinterval appears within an FFT buffer period 2309 as a result of thetime delay caused with respect to the second path 2306. Accordingly, thesamples of the OFDM symbol with respect to the first path 2308 are shownfor the FFT buffer 2309 which includes only samples of the OFDM symbol.However, as a result of the delay from the second path 2306 the FFTbuffer would include samples 2312 which are provided from the guardinterval. Correspondingly, for the signature sequence the samples fromthe first path 2308 are shown with respect to the samples of the secondpath 2306 which includes a component 2310 and the component 2311 fromthe OFDM symbol.

Correspondingly, FIG. 24 shows the effect of the second path 2306 of thechannel impulse response 2310. As shown in FIG. 24 the presence of theguard interval samples from the signature sequence 2310 causesinter-channel interference when the signals from the first and secondpaths 2306, 2308 are combined which represents inter-channelinterference 2402 for the detection of the signalling data from the OFDMsymbol. Equivalently in respect of the signature sequence, the presenceof the additional samples of the second path causes noise 2404 in thoseparts of the received signal which are affected by the presence of theguard interval within the FFT buffer.

According to the present technique in one example the transmitter isadapted to include a post fix circuit which adds a post fix formed fromthe samples of the guard interval to the preamble. An example is shownin FIG. 25 which is based on FIG. 15 but adapted to include a post fixcircuit 2501. In accordance with the present technique the samples of asignature sequence which form a guard interval are used to form a postfix signal which is fed to a corresponding gain unit 2503 and added tothe OFDM symbol to form a preamble symbol. The preamble symbol producedby the transmitter of FIG. 25 is illustrated in FIG. 26. The preamblesignal shown in FIG. 26 corresponds to the example shown in FIG. 22 inwhich the factor of P=0 and therefore the guard interval 2601 is madeentirely from samples of the signature sequence. With the value of Qequal to some value (a) then the component of the signature sequencecombined with the OFDM symbol is shown to form a fraction of thecomponent of the OFDM symbol 2602. The remaining part of the OFDM symbolis made from samples of the subcarriers which are conveying thesignalling data 2604. However, as shown in FIG. 26, as a result of thepresence of the post fix circuit 2501 and the gain adjustment circuit2503 the preamble symbol includes a post fix component 2606 which maycomprise all or some of the time domain samples of the signaturessequence which is used to form the guard interval 2601 as represented byan arrow 2610.

According to the present technique a receiver can then detect thesignalling data and the signature sequence in the presence of asignificant echo path which causes the inter-channel interference andsignature sequence noise showing in FIG. 24. As shown in FIG. 27a theFFT window position 2701 is shown for a first path 2703 and a secondpath 2705. In correspondence with the present embodiment the preamble2707 includes a guard interval 2709 which is made from the samples ofthe signature sequence and a post-fix 2711, which is formed from a partor all of the samples of the guard interval samples, which samples arethemselves formed from the signature sequence samples.

As shown in FIG. 27a , after having generated an estimate of the channelimpulse response 2714 which includes the two paths 2703, 2705, areceiver can regenerate the components of the post-fix 2711.1, 2711.2using the corresponding samples of the signature sequence which wereused to form the post-fix 2711. By combining the re-generated components2711.1, 2711.2 according to the channel impulse response 2714, andsubtracting the combined components from the received signal, a signalis formed as shown in FIG. 27 b. As can be seen in FIG. 27b , a part ofthe samples of the signalling OFDM symbol are formed 2720, but outsidethe FFT window 2701. Furthermore as shown in FIG. 27c , the FFT window2701 does not include a section 2722 of the OFDM symbols samples, whichare required to recover the signalling data. Accordingly, by copying thesignal samples 2720 to the position 2722 as represented by an arrow2724, the received signal shown in FIG. 27d is formed from which thesignalling data can be recovered.

A further example embodiment of the present technique is shown in FIG.28. The illustration shown in FIG. 28 corresponds substantially to theexamples shown in FIGS. 10, 13 and 20 so corresponding features have thesame numerical designations. In correspondence with the example shown inFIG. 20 the preamble is formed from the first 8K OFDM symbol 2801, whichis arranged to carry the signalling data and to which a signaturesequence T-SigSeg0 2802 or T-SigSegl 2803 is to be combined. However,the example embodiment shown in FIG. 28 is adapted to address a furtherimprovement with respect to the example preamble shown in FIG. 26. Thepreamble shown in FIG. 28 also has a post-fix as well as a guardinterval forming a pre-fix. However, it has been identified that if thepost-fix signal samples and the pre-fix signal samples are the same,then at the receiver may mistake the post-fix as the guard interval andattempt to recover the signalling data from the wrong samples of theOFDM symbol. The receiver may detect the EWS indication using a matchedfilter having and impulse response corresponding to the samples of theguard interval, which is therefore looking for the pre-fix. In fact twomatch filters are used to filter the samples of the preamble, a firstwhich has an impulse response matched to the signal samples of thesignature sequence used to form the guard interval or pre-fix with EWSon (T-SigSeq1) and another matched filter with an impulse responsematched to the signal samples of the signature sequence used to form theguard interval or pre-fix with EWS off (T-SigScq0). As soon as one ofthe matched filters detects the guard interval/pre-fix then decodingbegins of the preamble which follows in order to acquire additional EWSinformation, which is provided by the signalling data carried by theremainder of the OFDM symbol. This is in order to minimise a time todetect EWS. However, if the guard interval/pre-fix is the same as thepost-fix and the receiver turns on between the pre-fix and the post-fixbecause it has detected the post-fix by mistake, because the post-fixhas the same samples of the signature sequence as the guardinterval/pre-fix then because it detected the post-fix instead it has noway of knowing that the following 8K samples are not the preamblesymbols until it decodes and determines that detection of the signallingdata fails, for example a CRC or error correction decoding fails, or theintegrity of the data does not correspond to the pre-determined expectedformat.

Accordingly, this would lengthen the time to detect the EWS indication.According to the present technique a transmitter shown in FIG. 28 isadapted to form the guard interval/pre-fix with different samples of thesignature sequence then the post-fix. Therefore, according to thepresent technique one part of the signature sequence is used to form theguard interval/pre-fix and a different part of the signature sequence isused to form the post-fix and the remaining or a further part of thesignature sequence is combined with the OFDM symbol forming thepreamble. As shown in FIG. 28 in one part of the signature sequence to2804 is used to form the guard interval 2806 with a gain of 1/√Q asrepresented by an arrow 2808. As for the example shown in FIG. 26 nocomponents of the signal samples of the body of the OFDM symbol areincluded in the guard interval. The post-fix 2810 is formed from thesamples of the signature sequence 2812 from an earlier part of thesignature sequence samples which are added to the OFDM symbol. Byarranging for the samples of the guard interval in the samples of thepost-fix of the signature sequence to be taken from opposite ends of thesignature sequence samples which are combined with the body of the OFDMsymbol, there is a reduced likely hood of the post-fix being confusedwith the guard interval/pre-fix at the receiver and accordingly theaforementioned problem of miss detecting the EWS information provided inthe signature sequence is less likely.

A corresponding example is shown for the samples of the signaturesequence for indicating that the EWS is on 2803 (T-SigSeq1) with samplesof the signature sequence 2814 being copied to form the guard interval2816 as represented by an arrow 2815 with a gain of 1/√Q and samples ofan earlier part of the signature sequence 2818 being formed into thepost-fix 2820 as represented by an arrow 2817 with a gain of 1/√Q withremaining samples of the signature sequence 2822 being combined with theOFDM payload carrying samples as explained with reference to FIGS. 21and 26. As for the example shown in FIG. 26, the factor of P=0 andtherefore the guard interval 2601 is made entirely from samples of thesignature sequence. With the value of Q equal to some value (a) then thecomponent of the signature sequence combined with the OFDM symbol formsa fraction of the component of the OFDM symbol.

The following numbered clauses definme further example aspects andfeatures of the present technique:

1. A transmitter for transmitting payload data using OrthogonalFrequency Division Multiplexed (OFDM) symbols, the transmittercomprising

-   -   a frame builder configured to receive the payload data to be        transmitted and to receive signalling data for use in detecting        and recovering the payload data at a receiver, and to form the        payload data with the signalling data into frames for        transmission,    -   a modulator configured to modulate a first OFDM symbol with the        signalling data and to modulate one or more second OFDM symbols        with the payload data,    -   a signature sequence circuit for providing a signature sequence,    -   a combiner circuit for combining the signature sequence with the        first OFDM symbol,    -   a prefixing circuit for prefixing a guard interval to the first        OFDM symbol to form a preamble, and    -   a transmission circuit for transmitting the preamble and the one        or more second OFDM symbols, wherein the guard interval is        formed from time domain samples of part of the signature        sequence.

2. A transmitter according to clause 1, wherein the guard intervalincludes only the time domain samples of the part of the signaturesequence.

3. A transmitter according to clause 1 or 2, wherein the amplitude ofthe samples of the part of the signature sequence which are combinedwith the first OFDM symbol are less than the amplitude of the samples ofthe first OFDM symbol which are produced by modulating the sub-carrierswith the signalling data.

4. A transmitter according to clause 1, 2 or 3, wherein the prefixingcircuit is configured to form the guard interval from one part of thetime domain samples of the signature sequence and the transmittercomprises a post fixing circuit which is configured to add another partof the time domain samples of the signature sequence as a post fix tothe first OFDM symbol, the preamble comprising the guard interval withthe time domain samples of the one part of the signature sequence as apre-fix to the first OFDM symbol and the post-fix comprising the timedomain samples of the other part of the signature sequence, the timedomain samples of the one part of the signature sequence being differentfrom the time domain samples of the other part of the signaturesequence.

5. A transmitter according to clause 4, wherein the combiner circuit isconfigured to combine the time domain samples of the signature sequenceor a part of the signature sequence with the first OFDM symbol, and theprefixing circuit is configured to form the guard interval with the timedomain samples of the one part of the signature sequence from sampleswhich are copied from the time domain samples of the signature sequencewhich are combined by the combiner circuit with the first OFDM symbol,and the post fixing circuit is configured to add the other part of thetime domain samples of the signature sequence as a post fix to the firstOFDM symbol, from samples which are copied from the time domain samplesof the signature sequence which are combined by the combiner circuitwith the first OFDM symbol.

6. A transmitter according to any of clauses 1 to 5, wherein thesignature sequence processor circuit is a pseudo random binary sequencegenerator, an M-sequence generator or a Gold code sequence generator.

7. A transmitter according to any of clauses 1 to 6, wherein the messageprovided by the selection of the signature sequence is an indication ofa presence of an early warning signal.

8. A transmitter according to any of clauses 1 to 7, wherein thesignature sequence processor circuit includes a pseudo random binarysequence generator comprising a linear feedback shift register forin-phase samples (I) or quadrature phase samples (Q), and a generatorpolynomial for the linear feedback shift register for the in-phase andquadrature samples are selected from the following:

x¹³ + x¹¹ + x + 1 x¹³ + x⁹ + x⁵ + 1 x¹³ + x¹⁰ + x⁵ + 1 x¹³ + x¹¹ + x¹⁰ +1

9. A transmitter according to any of clauses 1 to 8, wherein aninitialisation for the linear feedback shift register for in-phase (I)or quadrature phase samples (Q) is one of the following:

Initialisation (LSB first) 1111111111111 1110111011111 01101101101110101010101010

10. A receiver for detecting and recovering payload data from a receivedsignal, the receiver comprising

-   -   a detector circuit for detecting the received signal, the        received signal comprising the payload data, signalling data for        use in detecting and recovering the payload data, the signalling        data being carried by a first Orthogonal Frequency Division        Multiplexed (OFDM) symbol, and the payload data being carried by        one or more second OFDM symbols, and the first OFDM symbol        having been combined with a signature sequence and prefixed with        a guard interval comprising one part of the signature sequence        to form a preamble,    -   a synchronisation circuit comprising a matched filter having an        impulse response which has been matched to the signature        sequence with the effect that an output of the matched filter        generates a signal representing a correlation of the signature        sequence with the received signal, and    -   a demodulator circuit for recovering the signalling data from        the lirst OFDM symbol for recovering the payload data from the        second OFDM symbols, wherein the guard interval is formed from        the other part of the time domain samples of the signature        sequence, the receiver including    -   a matched filtering circuit comprising a guard interval duration        matched filter, the guard interval duration matched filter        having an impulse response formed from a predetermined portion        of time domain samples of the signature sequence, with the        effect that the guard interval duration matched filter generates        a signal based on a correlation of the predetermined portion of        time domain samples of the signature sequence with a portion of        the received signal corresponding to the guard interval, such        that the matched filtering circuit can detect the signature        sequence from which the guard interval of the received signal        has been formed and with which the first OFDM symbol has been        combined.

11. A receiver according to clause 10, wherein the guard intervalincludes only the time domain samples of the part of the signaturesequence.

12. A receiver according to clause 10 or 11, wherein the amplitude ofthe samples of the signature sequence which are combined with the firstOFDM symbol are less than the amplitude of the samples of the first OFDMsymbol which are produced by modulating the sub-carriers with thesignalling data.

13. A receiver according to clause 10, 11 or 12, wherein the matchedfiltering circuit comprising one or more matched filters having animpulse response which is matched to a differentially encodedpredetermined portion of the time domain samples of a different one ofthe set of signature sequences, with the effect that an output of eachof the guard interval duration matched filters generates a signalrepresenting a correlation of the differentially encoded predeterminedportion of the time domain samples of one of the set of signaturesequences with a differentially encoded portion of the received signalcorresponding to the guard interval.

14. A receiver according to any of clauses 10 to 13, wherein thesignature sequence processor circuit is a pseudo random binary sequencegenerator, an M-sequence generator or a Gold code sequence generator.

15. A receiver according to any of clauses 10 to 14, wherein the messageprovided by the selection of the signature sequence is an indication ofan early warning signal.

16. A receiver according to any of clauses 10 to 15, wherein thesignature sequence is generated using a linear feedback shift registerfor in-phase samples (I) or quadrature phase samples (Q), and agenerator polynomial for the linear feedback shift register for thein-phase and quadrature samples are selected from the following:

x¹³ + x¹¹ + x + 1 x¹³ + x⁹ + x⁵ + 1 x¹³ + x¹⁰ + x⁵ + 1 x¹³ + x¹¹ + x¹⁰ +1

17. A receiver according to any of clauses 10 to 16, wherein aninitialisation for the linear feedback shift register for in-phase (I)or quadrature phase samples (Q) is one of the following:

Initialisation (LSE first) 1111111111111 1110111011111 01101101101110101010101010

Various further aspects and features of the present technique aredefined in the appended claims and various combinations of the featuresof the dependent claims may be made with those of the independent claimsother than the specific combinations recited for the claim dependency.Modifications may also be made to the embodiments hereinbefore describedwithout departing from the scope of the present technique. For instance,processing elements of embodiments may be implemented in hardware,software, and logical or analogue circuitry. Furthermore, although afeature may appear to be described in connection with particularembodiments, one skilled in the art would recognise that variousfeatures of the described embodiments may be combined in accordance withthe present technique.

1. A receiver for recovering payload data in a received signalcomprising Orthogonal Frequency Division multiplexed (OFDM) symbols, thereceiver comprising: circuitry configured to: detect, from the receivedsignal, OFDM symbols having modulated thereon a signature sequence, thereceived signal carrying physical layer signalling information whichindicates parameters for recovering the payload data by demodulation,the signature sequence representing a message, further information aboutthe message being conveyed to the receiver in the physical layersignalling information or the payload data, wherein the OFDM symbols ofthe received signal have a prefix formed of samples a first part of thesignature sequence; and use the signature sequence to synchronise thedemodulation to a frame of the received signal comprising preamble andpayload data symbols; and use the physical layer signalling informationin the preamble symbols to decode the payload symbols.
 2. The receiveras claimed in claim 1, wherein the message relates to properties of theinformation that are received in the payload data.
 3. The receiver asclaimed in claim 1, wherein the information in the payload data containsemergency information or software update information.
 4. The receiver asclaimed in claim 1, wherein the message relates to properties of theinformation that are received in the physical layer signallinginformation.
 5. The receiver as claimed in claim 1, wherein thesignature sequence has been chosen from a set of signature sequences,each signature sequence conveying a different message to the receiver.6. The receiver as claimed in claim 1, wherein the OFDM symbols have apostfix formed of samples of a second part of the signature sequence. 7.The receiver as claimed in claim 6, wherein samples of the first part ofthe signature sequence are not samples of the second part of thesignature sequence.
 8. The receiver as claimed in claim 1, wherein thephysical layer signalling information is detected from a preamble of thereceived signal.
 9. The receiver as claimed in claim 1, wherein thereceiver is configured to detect the message without first detecting thephysical layer signalling information.
 10. The receiver as claimed inclaim 1, wherein the receiver is configured to detect the messagewithout detecting a whole preamble of the received signal.
 11. Thereceiver as claimed in claim 1, wherein the receiver is configured todetect the message in a low power operating mode.
 12. The receiver asclaimed in claim 1, wherein the signature sequence and the prefix areused by the receiver to synchronise the demodulation to a frame of thereceived signal comprising preamble and payload data symbols.
 13. Thereceiver as claimed in claim 1, wherein the signature sequence is usedby the receiver to synchronise the demodulation to a frame of thereceived signal comprising preamble and payload data symbols bycalculating frequency offset.
 14. The receiver as claimed in claim 1,wherein the signature sequence is formed from two sequences output bytwo sequence generators.
 15. The receiver as claimed in claim 1, whereinthe receiver comprises a signature sequence generation circuit and acorrelation circuit to identify the message by correlating a signaturesequence generated by the signature sequence generation circuit with thereceived signal.
 16. The receiver as claimed in claim 1, wherein thesignature sequence represents a message indicating the presence of anearly warning signal or the message represents the absence of an earlywarning signal.
 17. The receiver as claimed in claim 16, wherein thesignature sequence representing a message indicating the presence of anearly warning signal is different to the signature sequence representinga message indicating the absence of an early warning signal.
 18. Thereceiver as claimed in claim 8, wherein the receiver comprises a matchedfilter configured to correlate a receiver generated signature sequencewith the received signal, the length of the match filter being shorterthan the number of samples of the preamble.
 19. A television receivercomprising the receiver according to claim 1 and configured to decodeaudio/video data from payload data.
 20. A method for recovering payloaddata in a received signal comprising Orthogonal Frequency Divisionmultiplexed (OFDM) symbols, the method comprising: detecting, from thereceived signal, OFDM symbols having modulated thereon a signaturesequence, the received signal carrying physical layer signallinginformation which indicates parameters for recovering the payload databy demodulation, the signature sequence representing a message, furtherinformation about the message being conveyed to the receiver in thephysical layer signalling information or the payload data, wherein theOFDM symbols of the received signal have a prefix formed of samples afirst part of the signature sequence; using the signature sequence tosynchronise the demodulation to a frame of the received signalcomprising preamble and payload data symbols; and using the physicallayer signalling information in the preamble symbols to decode thepayload symbols.
 21. The method as claimed in claim 20, wherein the OFDMsymbols have a postfix formed of samples of a second part of thesignature sequence.
 22. The method as claimed in claim 21, whereinsamples of the first part of the signature sequence are not samples ofthe second part of the signature sequence.
 23. The method as claimed inclaim 20, comprising detecting the message without first detecting thephysical layer signalling information.
 24. The method as claimed inclaim 20, comprising detecting the message in a low power operatingmode.
 25. The method as claimed in claim 20, comprising using thesignature sequence to synchronise the demodulation to a frame of thereceived signal comprising preamble and payload data symbols.
 26. Themethod as claimed in claim 20, comprising using the signature sequenceto synchronise the demodulation to a frame of the received signalcomprising preamble and payload data symbols by calculating frequencyoffset.
 27. The method as claimed in claim 20, wherein the signaturesequence is formed from two sequences.